The invention relates to color image display systems which employ one or more electro-optic projection-type image display devices. Such a display device serves as a light modulator, either in the transmissive or reflective mode, to control the grey level of projected light at each pixel point. More particularly, the invention relates to such a color display system having a reflective liquid crystal (LC) light valve and a polarizing beam splitter for separating the incoming and reflected light.
Color display systems are known in which light bars of different colors are sequentially scrolled across a single electro-optic light modulator panel to produce a color display. See, for example, commonly assigned U.S. Pat. No. 5,532,763, incorporated herein by reference.
These display systems are particularly suitable for displaying color images in successive frames, such as color video information, wherein each frame is composed of component color sub-frames, e.g., red, green and blue sub-frames.
These systems employ an electro-optic (LCD) light-transmissive or reflective modulator panel comprised of a row-and-column matrix array of pixels, for modulating the transmission of light in accordance with the image information signals during successive frame periods. The analog signal information is applied to the pixel columns of the array, a row at a time, during each frame period.
A similar system is also disclosed in the publication of J. A. Shimizu, xe2x80x9cSingle Panel Reflective LCD Projectorxe2x80x9d, Projection Displays V, Proceedings SPIE, Vol. 3634, pp. 197-206 (1999), which is incorporated herein by reference. Other references which disclose such a system are the published Japanese Patent Application Nos. 09,292,160 and 10,091,886; U.S. Pat. Nos. 5,914,817 and 5,946,054; International Publication No. WO 95/13561 (Application No. PCT/US94/12289) and IBM Technical Disclosure Bulletin Vol. 40, No. 12, PP 165-167. In this type of system, a reflective LCD is used to create the image. The active area on the LCD from which the picture information is reflected consists of an array of pattern aluminum mirrors. The reflective mirrors are formed on top of the array, covering the address circuitry and pixel transistors.
An incoming illumination beam is directed to a polarizing beam-splitter or xe2x80x9cPBSxe2x80x9d. The PBS directs a polarized beam onto the reflective LCD panel, which modulates the polarization with the picture information. The reflective LCD, in combination with the PBS, directs the picture forward through a projection lens and onto a screen.
Of all the components in the system, only the LCD panel, the PBS and the projection lens are in the imaging path. These components are therefore critical to the quality of the projected image. A typical PBS has very good polarizing contrast or xe2x80x9cextinctionxe2x80x9d over a small angular range of about +/xe2x88x921xc2x0; however, a wider angular acceptance, up to +/xe2x88x9212xc2x0, is possible only with a trade-off in extinction.
In order to produce a high contrast image, it is necessary for the PBS to have a high selectivity between the eigen-polarizations (xe2x80x9cpxe2x80x9d and xe2x80x9csxe2x80x9d) for the transmissive as well as the reflective case. For this reason, pre-polarizers and/or post-polarizers are used to restore the selectivity and, consequently, optical system contrast for the wide cone angles that are common in projection systems.
Commonly, sheet-type polarizers are used for this purpose, as they have very high selectivity, occupy little space and are relatively inexpensive. Grid polarizers may also be used. However, when sheet or grid polarizers are placed in the light path, they produce an eigen-polarization field that is generally different than the eigen-polarization field of the PBS. Since the two fields do not match, the resulting selectivity is reduced and the system contrast is diminished.
It is a principal object of the present invention to provide a color image display system which includes a reflective liquid crystal light modulator and a polarizing beam-splitter (PBS), arranged in front of the light modulator, for separating incoming and reflected light, which avoids the problems described above.
This object, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by providing a particular arrangement of the pre-polarizer or post-polarizer and the polarizing prism (PBS) wherein the fields of both polarizer types can be made the same and, thus, the system contrast can be improved.
More particularly, the present invention comprises:
(a) a light source producing a beam of light along a first axis;
(b) a reflective light modulator having an array of addressable pixels for modulating the polarization of light reflected from the pixels in accordance with display signals applied to the pixels, the array of pixels of the light modulator forming a substantially planar first surface arranged to receive and reflect the beam of light in a direction substantially perpendicular to the first surface;
(c) an electronic circuit for repeatedly addressing the array of pixels of the light modulator with display signals corresponding to a picture image;
(d) a projection lens having an optical axis aligned along a second axis, the projection lens being arranged to receive and project light onto a display screen, the second axis intercepting the first axis;
(e) a polarizing beam-splitter (PBS), having a reflective/transmissive surface with a normal n, interposed on both the first and second axes to intercept the beam of light from the source and to pass the beam to the light modulator, and to intercept the reflected light from the light modulator and to pass the reflected light with a given polarization to the projection lens along the second axis; and
(f) a polarizer arranged along one of the first and second axes to receive and polarize the beam of light, the polarizer having a polarizer absorbing axis which extends parallel to the normal n.